The present invention relates to a frequency converting device, and more particularly to a frequency converter applied to a speed control (revolution number control) of an ac motor, e.g. an induction motor for use in an air conditioner, refrigerating device or the like.
For control of the revolution speed of an ac motor, e.g. an induction motor, there has been widely used a frequency converting device comprising in combination an inverter and a converter. In an air conditioning system, such as a home air conditioner, the refrigerating device or the like often utilizes the frequency converting device. For instance, the home air conditioner realizes an optimum air conditioning performance in responsive to the air conditioning load to accomplish improved comfort and energy savings.
Meanwhile, a commercial ac power supply for feeding power to a frequency converting device has different voltage values. For example, they are typically 100 V and 200 V in Japan. In some other countries, typical voltage values are 115 V and 230 V. For this reason, different types of air conditioners, refrigerating devices or the like had to be provided in accordance with the voltage values of the commercially available power supplies. This has been an obstacle in improving the efficiency of manufacture, sale, etc.
On the other hand, frequency converting devices with PWM (pulse width modulation) control system have heretofore been widely known. For instance, U.S. Pat. No. 4,410,937 to Michika Uesugi teaches a technique in which a signal for controlling a three phase inverter is stored in a ROM (read only memory) of a relatively small capacity. Further, in the paper entitled "Relationship between output waveform of PWM inverter and gate control signal" by Takahashi et al., (Journal of Japanese electric society Vol. 95, No. 2, Feb., 1975), there is exhibited a theoretical analysis in regard to improvement in the relationship between output waveform of a PWM inverter and the load terminal voltage or output voltage waveform, etc. However, these prior teachings do not take into consideration the adjustment of the inverter output in accordance with voltage values of the commercial power source.
There is also known a method of converting a voltage value of a power source into a desired value by using a transformer. However, because this method requires the transformer, the device is large and heavy, and has a large energy loss.